In a digital communication system, the presence of noise can cause a receiver to make incorrect decisions about the data that a digital communication signal is conveying, resulting in a higher bit error rate (BER) than might be experienced in the presence of less noise. If the influence of noise can be reduced in a receiver, then BER improvements can result or other digital communication system parameters can be altered while maintaining an acceptable BER. Among others, such parameter alterations include transmission power level reductions, component cost reductions, and/or higher data rates.
Generally, two types of noise exert a significant influence in a digital communication receiver. The first type of noise is primarily additive because it affects the amplitudes of signals propagating in a communication system. Thermal noise, also known as Johnson noise or Nyquist noise, and interference primarily lead to additive noise. Additive noise is the subject of the signal-to-noise ratio parameter that is often used to characterize digital communication systems.
A second type of noise, called phase noise, is primarily temporal because it is characterized by fluctuations in the phase of various alternating signals used in a communication system. The majority of phase noise typically results from the use of oscillators to generate signals for up-conversion and down-conversion operations. As a general rule, much more phase noise results from the use of tunable oscillators than from the use of fixed frequency oscillators. However, tunable oscillators are often viewed as a practical necessity in communication systems that use frequency division multiplexing (FDM) and allow transmitters and receivers to tune to different frequency channels. Moreover, as a general rule less expensive oscillators generate more phase noise than more expensive oscillators.
FIG. 1 shows a simplified block diagram of a conventional digital communication receiver 10. Referring to FIG. 1, a received digital communication signal 12 is inevitably characterized by some amount of phase noise due to an up-conversion process at a transmitter (not shown) and some amount of additive noise due to thermal noise present in the transmitter and to various types of interference. Received signal 12 is down-converted in a mixer 14 using an oscillation signal provided by an oscillator 16. When oscillator 16 is frequency-tunable, as is often desirable in FDM communication systems, a significant amount of phase noise may be added to the already-present phase noise. Moreover, additive noise further accumulates due to the thermal noise associated with mixer 14 and other receiver front-end components that are omitted from FIG. 1. The down-converted signal is conventionally digitized at a block 18, possibly filtered through an optional adaptive equalizer 20, and passed to a conventional carrier tracking loop 22. The output of carrier tracking loop 22 drives a decoder 24 which extracts data from the received digital communication signal.
Carrier tracking loop 22 is a phase locked loop that includes a phase rotator 26 to receive the down-converted digital communication signal. A rotated signal portion of the carrier tracking loop signal from phase rotator 26 may be filtered through an optional adaptive equalizer 28, and a resulting equalized portion of the carrier tracking loop signal passed to a phase constellation error detector 30. Typically, either adaptive equalizer 20 or adaptive equalizer 28 is included in receiver 10, with adaptive equalizer 28 achieving better results in equalizing channel characteristics. A phase constellation error signal portion of the carrier tracking loop signal is passed from error detector 30 through a loop filter 32 to a phase integrator 34, which feeds a phase-conveying signal back to phase rotator 26.
In prior art digital communication receivers, such as receiver 10, reductions in the influence of phase noise lead to increases in the influence of additive noise, and vice-versa. Conventionally, the influences of additive and phase noises are balanced against one another in carrier tracking loop 22. A loop bandwidth parameter established primarily by loop filter 32, is set to desirably balance anticipated additive noise effects with anticipated phase noise effects. Generally, a narrower loop bandwidth is used to minimize the influence of additive noise, but narrowing the loop bandwidth impedes the carrier tracking loop's ability to track phase noise. A wider loop bandwidth is used to minimize the influence of phase noise, but widening the loop bandwidth increases out-of-band energy in the received signal and allows additive noise to assert a greater influence.
In some applications, conventional carrier tracking loop 22 risks complete failure. This occurs when signal-to-noise requirements in the down-converted digital communication signal dictate a bandwidth so narrow that phase noise cannot be tracked or when phase noise tracking requirements dictate a bandwidth so wide that the signal-to-noise ratio becomes too small to successfully extract communicated data. The conventional solution to this dilemma calls for any of a variety of component enhancements, which can dramatically increase the cost of receiver 10. For example, a high quality oscillator 16 which generates reduced phase noise can be used, but such a component may cost many times more than an alternate oscillator that generates more phase noise.
The phrase "modulation order" refers to the number of bits that are communicated in each unit interval of a digital communication signal. For example, QPSK has a modulation order of two and transmits two bits per unit interval, and sixteen-QAM has a modulation order of four and transmits four bits per unit interval. Modern digital communication systems achieve improved data rates through the use of higher modulation orders, such as a rate of four or more bits per unit interval. However, higher modulation orders typically require moderately high signal-to-noise ratios. A reduction in the influence of phase noise on a digital communication receiver would allow the balance between phase noise and additive noise achieved in the carrier tracking loop to favor higher signal-to-noise ratios and higher modulation order communication. Likewise, a reduction in the influence of phase noise on a digital communication receiver would result in a lowered BER. Or, it could allow the use of FDM operations and/or inexpensive components, particularly oscillators, while maintaining an acceptable BER.